This invention is related to systems for detecting fires or explosions in hazardous areas by detecting changes in the ambient conditions in the vicinity of the explosion.
When working in underground coal mines, sanitary sewers or other underground networks where flammable gases can accumulate, it is extremely important that one be able to quickly detect the occurrence of an explosion and erect some type of barrier to contain the explosion. An effective means of containing explosions in an underground coal mine is to provide a triggering device in the mine shaft upstream (on the explosion side) of a barrier so that when the triggering device senses conditions indicative of an explosion, the barrier may be triggered before the flame propagating from the explosion passes beyond it. The barrier may be of the type described in U.S. Pat. No. 3,958,644 in which a quantity of explosive suppression agent, usually water, is released from an overhead container. A variety of devices for triggering such explosion barriers have been developed.
Some triggering devices utilize thermocouples to sense directly the temperature rise of an explosion but such devices are limited to sensing the flame at a single point in space and may trigger late when the flame front does not fill the entire cross section of the mine shaft. Furthermore, they are slow in response and, therefore, trigger late unless extremely fine thermocouple wires are used in which case they are fragile and easily damaged by shocks and impacts. Finally, they may be falsely triggered by incidental flames or heat sources such as welding torches, etc.
A second type of device detects ultraviolet, visible, infrared or black body radiation emitted by the explosion flame, but these devices can be triggered by one or more false signals generated by miner's cap lamps, lights on vehicles or equipment for illumination, sparks or arcs and hot surfaces.
A third type of device may comprise a wind vane which responds to dynamic wind forces generated ahead of an explosion, but such devices are slow to respond, are sensitive to shocks and impacts, and can respond falsely to a roof fall or blasting concusion. Also, they may fire prematurely during a dust explosion since there is little or no correlation between dynamic wind force and flame location.
A fourth type of triggering device responds to the static pressure rise in the mine shaft, but such devices suffer from the same false and disadvantages as the dynamic wind-type devices discussed above.
Finally, a triggering system has been proposed in which a static pressure sensor is used to arm an infrared flame sensor which triggers the explosion barrier when the flame is detected. Such a system is described by D. B. Lull et al, "Development of a System to Suppress and Extinguish Fully Developed Coal Dust Explosions" Final Report, NSWC/DL Technical Report TR-3151, Feb. 1975. When a radiation source is within the view of the flame sensor, the proposed pressurearm trigger device may be falsely fired by a static pressure rise generated during either blasting or a roof fall or the device may be prematurely fired during a dust explosion. The above-cited report states, on page B-11 thereof, the possibility of utilizing multiple detectors for decreasing the probability of false firing, but does not elaborate further.
For maximum effectiveness, the explosion suppressant should be ejected in front of the approaching flame. If the trigger signal if premature, the suppressant will be driven downstream and its concentration diluted by the wind forces prior to be overtaken by the flame and, therefore, the efficiency of the suppressant will be decreased and explosion suppression will be delayed or not obtained at all. On the other hand, if the trigger signal is late, the suppressant will be delivered in back of the flame and the explosion will not be suppressed.